Containers for foodstuffs are generally produced by conventional thermoforming processes. These processes typically involve feeding a continuous thermoformable polymeric sheet from a feed spool or extruder through a thermoformer where a thermoformed container is formed in the sheet, and subsequently cutting the containers from the sheet. In some processes, the container is filled and sealed prior to cutting the container from the sheet. In other processes, the containers are cut from the sheet and then transported to another location for filling.
FIG. 1A illustrates a conventional thermoforming process. In this exemplary process, a polymeric sheet 1 is fed through a thermoformer 2 where thermoformed containers 3 are formed into the sheet 1. The thermoformed containers 3 are then cut from the sheet 1 by a cutting die 4. The cut containers 6 may then be sent via a conveyor to a packaging station or directly to a filling station to be filled with a product. The resulting waste sheet 5 is then collected and reprocessed.
As illustrated in FIG. 1B, conventional thermoforming processes generate a substantial amount of waste. The waste sheet 5 often comprises a substantial amount of leftover web material from sheet 1. Typically, only 55 to 60 percent of sheet 1 is utilized. The remaining 40 to 45 percent is waste and is recycled. This underutilization of the sheet increases production costs by increasing recycling and extrusion demands. Also, the amount of recycle material used when extruding the sheet 1 may be limited to control the appearance and performance of the formed containers.
As mentioned previously, in many cases, the formed containers are shipped to a different location for filling. The packaging must be prepared in such a way to protect the containers during shipment. Moreover, trucks and sea containers are typically maxed out on the volume of the containers rather than the weight. Accordingly, shipping costs for preformed containers may be high due to the low container bulk density and the high cost of protective packaging material.
Furthermore, containers produced by conventional thermoforming processes tend to have a relatively high dimensional variability between containers. There are factors that negatively affect the dimensional consistency and overall quality of containers produced by conventional thermoforming processes. First, the processing window for the polymeric sheet 1 may have a tolerance of only a few degrees due to the semi-crystalline nature of some polymers, such as polypropylene. Also, the inherent low melt strength of some polymers such as polypropylene may result in the sheet sagging as it reaches forming temperature.
The multi-lane design of the thermoformer may also contribute to the dimensional variability between containers. Referring back to FIG. 1A, the sheet 1 travels in the machine direction (i.e., from left to right in FIG. 1A). As shown in FIG. 1B, the sheet has two edges 7, 8.
Due to the edge effects, the outboard cavities (i.e., the mold cavities used to form containers with material proximal to the edges 7, 8) tend to be cooler than the center cavities. Similarly, the two dimensional tooling array results in cooling inconsistencies across the tool. Moreover, the intermittent motion of the thermoforming process may result in additional temperature inconsistencies between containers formed in the same lane in the machine direction.
Accordingly, it would be desirable to provide a new thermoforming process which allows for more efficient utilization of the polymeric sheet. It would also be desirable to provide new thermoforming processes which reduce the costs associated with transporting container materials. Furthermore, it would be desirable to provide new thermoforming processes which produce consistent cavity-to-cavity quality containers.